Spectroscopy is a general term for the process of measuring energy or intensity as a function of wavelength in a beam of light or radiation. Many conventional spectrometers, and components comprising a spectrometer system, also referred to as an instrument, may include basic features and components such as a slit and a collimator for producing a parallel beam of radiation, one or more prisms or gratings for dispersing radiation through differing angles of deviation based on wavelength, and apparatus for viewing dispersed radiation. Spectroscopy uses absorption, emission, or scattering of electromagnetic radiation by atoms, molecules or ions to qualitatively and quantitatively study physical properties and processes of matter.
Light or radiation directed at a target, or sample of physical matter, during operation of a spectrometer system may be referred to as incident radiation. Redirection of incident radiation following contact with a sample of physical matter (“sample”) commonly is referred to as scattering of radiation. To the extent that atoms or molecules in a sample absorb all or a portion of incident radiation, rather than reflect incident radiation, a sample may become excited, and the energy level of the sample may be increased to a higher energy level. Electromagnetic radiation, including incident radiation, that passes through a sample, may produce a small portion of light that is scattered in a variety of directions. Light that is scattered but continues to have the same wavelength as the incident radiation will also have the same energy, a condition often referred to as Rayleigh or elastically scattered light. Incident radiation that is scattered during a change of vibrational state in molecules may be scattered with a different energy, and such scattered light may be called Raman scattered light. Such phenomena have been used in conjunction with spectroscopy to qualitatively and quantitatively study physical properties and processes, including identification of chemical properties, compositions, and structures of a sample.
A wave associated with electromagnetic radiation may be described by wavelength, the physical length of one complete oscillation, and by frequency of the wave, the number of oscillations per second that pass a point. If incident radiation is directed at a sample, the wavelength of the incident radiation may remain substantially unchanged in scattered radiation. Alternatively, if incident radiation is directed at a sample, the wavelength in the scattered radiation may acquire one or more different wavelengths than the incident wavelength. The energy differential between the incident radiation and the scattered radiation may be referred to as a Raman shift. Spectroscopic measurement of Raman scattered light seeks in part to measure the resulting wavelength of such scattered light.
Raman scattered light may occur at wavelengths shifted from the incident light by quanta of molecular vibrations. The phenomenon of Raman scattered light, therefore, is useful in spectroscopy applications for studying qualities and quantities of physical properties and processes, including identification of chemical properties, compositions, and structure in a sample. Currently, Raman shift spectroscopic analytical techniques are used for qualitative and quantitative studies of samples. If incident radiation is used to scatter light from a sample, and scattered radiation data is measured, the scattered radiation may provide one or more frequencies associated with the sample, as well as the intensities of those shifted frequencies. The frequencies may be used to identify the chemical composition of a sample. If, for example, intensities are plotted on a Y-axis, and frequency or frequencies are plotted on an X-axis, the frequency or frequencies may be expressed as a wave number, the reciprocal of the wavelength expressed in centimeters. The X-axis, showing frequency or frequencies, may be converted to a Raman shift in wave numbers, the measure of the difference between the observed wave number position of spectral bands, and the wave number of radiation appearing in the incident radiation.
While these principles and phenomena are known, until recently efforts to apply the principles and phenomena to qualitative and quantitative analyses of samples have not always resulted in uniform, predictable results, or in acceptable levels of precision and accuracy of Raman spectra. Because of instrumentation variabilities, inherent weakness of a Raman scattered signal, fluorescence, and other limitations associated with spectroscopy instruments, the goal of producing a standard Raman spectrum for use in sample analyses was, until recently, a challenge not achieved by apparatus and methods known in the art.
At least one problem that had to be overcome was the fact that spectroscopic measurements of Raman scattered light seeking to measure wavelength or intensities, or both, of scattered light, could be affected by the instrument, or spectroscopic system, itself. A number of components of an instrument may contribute individually and collectively to undesirable instrumentation variabilities that affect spectral data measured by the instrument. Raman scattered radiation from a sample may be observed, measured, and directed through an instrument by optics of a spectrometer, may be coded by a device such as an interferometer, and may be directed to one or more detectors to record Raman spectra. Any one, or all, of such components of a conventional spectrometer system induced or contributed to instrumentation variabilities that reduced or adversely affected the precision and accuracy of measurements of Raman scattered light.
In addition to fluorescence, spectral measurements of a source of incident radiation such as a laser, including semiconductor or diode lasers, will evidence other varying baseline components, artifactual or real, that preferably could be eliminated, suppressed, or compensated for to provide an accurate Raman spectrum for analytical purposes. In instrumentation designs preferred by users of Raman technology, semiconductor diode lasers would be the choice of incident radiation due to small and compact sizing, low heat dissipation, and high energy conversion efficiency. Use of semiconductor or diode lasers, while useful because of a number of important characteristics, also engender unique problems that, if solved, would advance Raman technology. However, at least one other problem associated with semiconductor diode lasers is the tendency for the output to change from one frequency to another during operation, commonly referred to as frequency drift. Frequency drift is generally related to temperature variations that may cause either slow frequency drifts or drastic frequency changes. Semiconductor diode lasers also are susceptible to mode hops when the laser switches output from one frequency to a new preferred frequency.
Some of the problems associated with frequency shifts were discussed as early as 1991 in Semiconductor Diode Lasers Volume I, edited by William Streifer and Michael Ettenberg, IEEE Press (1991), a work incorporated by reference into this document. In general, frequency shifts, or mode hops, are inherent in laser light, and can be eliminated only by redesigning the laser at excessive cost. Solutions for overcoming the effects of frequency shifts have included redesign of the internal cavity of lasers, designing what is known as an external cavity for lasers, and tuning a range of modes into a single mode. All of those solutions are achieved at considerable expense, and generally shorten the useful life of a semiconductor laser.
A further problem related to diode lasers includes variations in output intensity that directly affect the measured Raman shift. Rather than eliminate the problem physically, which is expensive and limits the effective life of the laser, it would also be useful to compensate for the frequency shifts and intensity variabilities. Thus, it is at least an objective of the present invention to overcome problems associated with using excitation sources in the visible range of light, including, for example, removal of fluorescence and other common mode noise from acquired spectra. U.S. Pat. No. 6,281,971 issued to Allen et al., for example, attempts to solve problems related to frequency drift in semiconductor or diode lasers. The '971 patent, however, requires monitoring the laser output frequency and performing complex integration routines to obtain a Raman spectrum for a sample.
Raman scattering is a comparatively weak effect when compared with Rayleigh or elastic scattering. Nevertheless, Raman scattering offers a significant opportunity for qualitative and quantitative studies of physical properties and processes, including identification of chemical compositions and structure in samples of physical matter. To appreciate these phenomena, as well as understand the problems solved by the present invention, it should be noted that depending on the compound comprising a sample, only about one scattered photon in 10.sup.6-8 tends to be Raman shifted. Because Raman scattering, therefore, is such a comparatively weak phenomenon, an instrument used to disperse radiation for measurement purposes should have minimal stray light and be able to substantially reject Rayleigh scattering; otherwise, a Raman shift may not be measurable.
As earlier described, Raman phenomena result in spectral information that is shifted relative to the excitation source, or source of incident radiation. Thus, any variations in the excitation source will result in a relative change, or shift, in spectral information. Spectrally shifted Raman information also is directly related to the intensity of the excitation source. A further complication arises from multiple lines in the frequency of the source of incident radiation that may cause shifted, multiple sets of spectra from a sample. Therefore, conventional Raman experimentation discloses that a source or sources of incident radiation that causes or cause excitation in a sample used in connection with a spectrograph should be substantially monochromatic, preferably providing a single frequency or wavelength. Recognition that the source of incident radiation requires a substantially monochromatic frequency has led to use of a variety of laser light sources as a source of incident radiation because of the substantially monochromatic frequency and high intensity of a laser. Gas lasers such as helium-neon, helium-cadmium, argon-ion, krypton-ion, as well as solid state lasers including Nd-YAG, and diode lasers, solid state tunable lasers, liquid dye lasers, and other lasers, have been used.
An undesirable result of incident radiation on a sample occurs if a sample generates red shifted radiation as part of a radiation absorption process, a phenomenon commonly referred to as fluorescence. Fluorescence occurs when absorbed radiation is reduced in frequency by internal molecular processes and emitted as radiation that is closer to the red end of the visible light spectrum. Fluorescence sometimes may be strong enough in comparison with the Raman shift to swamp, or substantially eliminate, the weaker Raman signal. Fluorescence is a major interference for samples using excitation wavelengths in the visible region of the light spectrum, and has therefore made use of blue and green excitation sources problematic. Using excitation sources in the far end of the red end of the light spectrum mitigates the fluorescence effect, however, particularly in connection with silicon detectors, but substantially restricts use of instrument components that tend to provide radiation far into the infrared (“IR”) region of the light spectrum.